In a fiber optic communication system, a light wave is supplied to an input end of an optical fiber for propagation through the fiber. Regardless of the optical polarization of the wave at the input end of the fiber, by the time the wave reaches an output end of the fiber the polarization is random (unpredictable and uncontrollable). That is, the polarization of the light as it reaches the output end of the fiber in general will be randomly different from that at the input end thereof. This randomization of polarization is the result of randomly variable physical phenomena, such as birefringence due to mechanical strains in the fiber, that modify the optical polarization of light propagating through fiber. Accordingly, any optical amplifier device for amplifying the light exiting from the output end of the fiber should be capable of amplifying with equal gain any optical polarization; that is, the optical gain of the amplifier should be independent of the polarization, for otherwise the intensity of the light emanating from the amplifier will be undesirably randomly variable, as well as undesirably low in those cases where the light exiting from the fiber has a polarization for which the optical gain of the amplifier is low relative to its gain for the other polarization(s).
It is, of course, well known that any optical polarization can be viewed as a linear combination (a sum) of just two independent polarizations, such as TE and TM (transverse electric and transverse magnetic) waves, with a phase difference between them which may vary with time. Thus, an optical amplifier that has equal gain for just two independent polarizations will have equal gain for all polarizations.
In prior art, semiconductor laser (optical oscillator) structures have been proposed for use as optical amplifiers. A major problem with the resulting semiconductor optical amplifiers is an undesirable difference in optical gain for different polarizations: a semiconductor optical amplifier typically has a difference of several dB (decibels) in gain between TE and TM input waves. A typical semiconductor laser structure, with an active layer whose (rectangular) cross section is very non-symmetric, for example, about 0.15 .mu.m (micrometer) thick and about 2 .mu.m wide, has significantly different confinement factors--i.e., different fractions of optical energy confined to the active region, where optical amplification occurs--equal to about 0.4 for TE and 0.2 for TM waves having a wavelength equal to about 1.3 .mu.m. Hence, in general, a semiconductor optical amplifier whose structure is the same as that of the laser (optical oscillator) has undesirably different optical gains for TE and TM waves--for example, at least twice (0.4/0.2) the gain (3 dB) for TE than for TM waves in the case of the typical semiconductor optical amplifier structure for 1.3 .mu.m wavelength. Moreover, because of the difference in the indices of refraction between TE and TM waves, the optical reflectivity of the facets (end faces) of a semiconductor optical amplifier structure significantly depends upon the optical polarization of the light which is being reflected, whereby the difference between the optical gains for TE and TM waves can be undesirably increased.
In order to mitigate this problem, semiconductor optical amplifier devices have been proposed which involve the use of two optical amplifiers structures, in which the polarization dependence of one of the structures is undone (compensated) by that of the other, or which involve the use of a single laser amplifier structure in a double-pass configuration. These devices respectively are taught in a paper authored by G. Grosskopf et al, entitled "Optical Amplifier Configurations with Low Polarization Sensitivity," published in Electronics Letters, Vol. 23, pp. 1387-1388 (1987), and in a paper authored by N. A. Olsson, one of the inventors herein, entitled "A Polarization Independent Configuration Optical Amplifier," published in Electronic Letters, Vol. 24, pp. 1075-1076 (1988). However, the use of the two structures, although feasible, requires two semiconductor chips and hence would be undesirably costly because of its structural complexity; and the use of the double-pass configuration, although feasible, requires tight (careful marginal) control over the reflectivities of anti-reflection ("AR") coatings on the various elements of the configuration and hence likewise would be undesirably costly because of its complexity. Therefore there is a need for a polarization independent semiconductor optical amplifier device structure that is more compact than those of prior art and that does not require as tight control over the reflectivities of AR coatings.